How Often Should a Gas Furnace Be Maintained?
A gas furnace should be professionally serviced once a year by a licensed HVAC technician. Annual maintenance keeps the furnace running safely and efficiently, catches problems before they become breakdowns, and is essentially required to keep manufacturer warranties valid. For southern New Hampshire homes that run heat seven months a year, annual service is one of the highest-payoff things a homeowner can stay on top of.
Here is what an annual gas furnace tune-up actually includes and why each step matters.
When to schedule
The right window for furnace service is late summer through October, before the first cold snap of the season. Demand is lighter, scheduling is easier, and any problems found can be addressed before you actually need heat. Our schedule fills fastest right after the first 20°F night of the season.
What a complete gas furnace tune-up includes
A proper annual service covers a long checklist. The high-impact items:
Combustion analysis
A digital combustion analyzer measures the exact composition of the flue gases (CO, CO2, O2, stack temperature). Proper combustion should produce:
- CO under 50 parts per million in the flue (manufacturer specs vary)
- CO2 in the manufacturer's specified range (typically 8 to 10 percent for gas)
- Stack temperature in the manufacturer's specified range
Out-of-range combustion indicates burner adjustment is needed, the heat exchanger may be compromised, or the venting has an issue. This is the single most important safety check.
Heat exchanger inspection
The heat exchanger separates combustion gases from the air you breathe. Cracks or holes let CO into the home. A technician inspects visually and, when access allows, with a borescope. Heat exchanger failures are a hard-stop reason to red-tag and replace the furnace.
Burner inspection and cleaning
Burners can accumulate rust, dust, or spider webs that disrupt the flame pattern. Cleaning and adjustment restore proper combustion. A burner that produces a yellow or flickering flame instead of a steady blue is a clear signal something is wrong.
Ignitor and flame sensor
Modern furnaces use a hot-surface ignitor (a small ceramic element) instead of an old-style pilot light. Ignitors degrade over time and are a common cause of no-heat calls. A weak ignitor often shows up in pre-failure resistance testing. Replacement during a tune-up is cheap insurance against a January no-heat call.
The flame sensor is a small metal rod in the flame path that confirms ignition occurred. Oxidation on the sensor causes intermittent shutdowns. Cleaning takes minutes.
Blower assembly
- Confirm the blower wheel is clean (dust on the blades reduces airflow significantly)
- Inspect the blower motor for proper amperage draw
- Verify the capacitor is at the correct microfarad rating
- Listen for bearing noise that signals motor end-of-life
Air filter
Replace the filter or note when it needs replacement. A clogged filter is the most common reason a furnace overheats and shuts down on safety. We do not want the customer's January no-heat call to be a filter that should have been changed in October.
Gas pressure and connections
- Measure inlet and manifold gas pressure against manufacturer specs
- Inspect gas line connections for leaks using leak detection solution or a gas leak detector
- Verify the gas shut-off valve operates properly
Venting and intake
For high-efficiency condensing furnaces with PVC venting:
- Inspect both intake and exhaust pipes for blockages, separations, or animal nesting
- Verify the condensate drain is clear and the trap is functioning
- Check the venting slope
For non-condensing furnaces with metal venting:
- Inspect the flue for rust, separation, or blockage
- Verify the draft inducer is operating correctly
Safety controls and thermostat
- Test the high-limit switch
- Test the pressure switch (induced-draft furnaces)
- Test the flame rollout switch
- Verify thermostat operation and accuracy
Why annual service pays off
Safety
Gas furnaces produce carbon monoxide as a normal byproduct of combustion. Properly vented, none of it reaches living spaces. A cracked heat exchanger, blocked vent, or improper combustion can change that. Annual combustion analysis is the most reliable way to confirm safe operation.
Reliability
Most no-heat calls we respond to in midwinter have a root cause that would have been caught during routine service: a clogged filter, a weak ignitor, a sticky pressure switch, a flame sensor needing cleaning. Service in October is cheaper than an emergency call in January.
Efficiency
A properly tuned furnace can use 5 to 15 percent less fuel than the same unit with degraded combustion or restricted airflow. Across a full heating season, that adds up.
Warranty
Most major furnace manufacturers (Bosch, Trane, Lennox, Goodman, Carrier, Rheem) require documented annual professional service to maintain warranty coverage. Skipping a year can void coverage on a several-thousand-dollar repair.
What homeowners can do between visits
- Check or change the air filter monthly
- Keep at least 30 inches of clear space around the furnace
- Listen for unusual noises during operation
- Watch for any soot or scorching near the burner area
- Test CO detectors monthly and replace batteries annually
Schedule a tune-up
If your gas furnace has not been serviced in over a year, or you want to get on a regular maintenance schedule, contact A.J. LeBlanc Heating. Serving New Hampshire families since 1928.
Frequently Asked Questions
How often should I service my gas furnace?
Once a year, ideally in late summer or early fall before the heating season begins.
Is annual service required for the warranty?
For most major manufacturers, yes. Warranties typically require documented annual professional service to remain valid.
What is the best time to schedule furnace maintenance?
September and October. Demand is lighter and any problems found can be addressed before you actually need heat.
Can I service my furnace myself?
Filter changes and visual inspections, yes. Combustion analysis, gas pressure adjustment, heat exchanger inspection, and electrical work require a licensed technician with proper instruments and (in NH) a gas fitter license.
How long does a tune-up take?
A complete tune-up typically takes 45 to 90 minutes depending on the system, accessibility, and what is found during the inspection.